The present invention relates generally to code division multiple access (CDMA) cellular communications systems. More specifically, but without limitation thereto, the present invention relates to assigning variable length codes to service a maximum number of users in a code division multiple access service area that is assigned a fixed frequency band.
The radio frequency spectrum is limited almost everywhere in the world and is generally licensed in fixed frequency bands. Code division multiple access (CDMA) cellular communications systems typically use a set of CDMA codes that are orthogonal to one another to avoid mutual interference. The term orthogonal is applied to a set of codes if the vector dot product of any code in the set with any other code in the set results in zero. For example, the code (−1, −1) is orthogonal to the code (−1, 1) since −1*−1+−1*1=0. Currently known orthogonal CDMA codes have a code length of 2n, where n is a positive integer. For example, if each antenna element receives a signal having a bandwidth of 5 MHz and the available bandwidth for the feeder link is 500 Mhz, then the ratio of total available feeder link bandwidth to element bandwidth is 100, which is not a power of 2. In this example, the parameters of available feeder link bandwidth and antenna element bandwidth are difficult to change. The element bandwidth is dictated by the third generation wireless standards, and the feeder link bandwidth is assigned by a regulatory agency such as the Federal Communications Commission. Using conventional CDMA codes, the longest code length that could be used in this example is 64. The resulting feeder link bandwidth would therefore be 320 MHz, resulting in an unused bandwidth of 180 MHz. The unused bandwidth represents a loss of revenue from potential CDMA subscribers that might otherwise be included in the same service area.